1. Field of the Invention
The present invention relates to a vibration isolating apparatus which prevents transmission of vibration from a vibration-generating member, and particularly to a vibration isolating apparatus which is applicable to a kind of mount supporting an engine mounted on a vehicle.
2. Description of the Related Art
Heretofore, a vibration isolating apparatus used as an engine mount is disposed between an engine which is a vibration-generating portion of a vehicle and a vehicle body which is a vibration-receiving portion. The vibration isolating apparatus absorbs vibration generated by the engine and prevents the vibration from being transmitted to the vehicle body.
However, when a vibration isolating apparatus having a simple structure damps vibration in a low frequency range where shake vibration is generated (which is hereinafter referred to as a shake vibration range), a dynamic spring constant of an idle vibration range which is a frequency range higher than shake vibration range inversely increases, vibration in an idle vibration range cannot be sufficiently reduced. On the contrary, when the dynamic spring constant in the idle vibration range is decreased, vibration in the shake vibration range cannot be sufficiently damped.
Accordingly, there has been discussed a structure having a plurality of orifices in order to solve the above-described problems.
For example, there has been proposed a vibration isolating apparatus 200 as shown in FIG. 6.
The vibration isolating apparatus 200 is constructed as follows. An elastic body 116 is disposed between a bottom plate 110 having a bolt 112 of the vibration isolating apparatus 200, and a top plate 118 with a bolt 120 and a connecting plate 119. The top plate 118 and the connecting plate 119 are connected with each other by welding, so that an end portion of a flange portion 119A of the connecting plate 119 abuts against the elastic body 116.
A main liquid chamber 126 and a first sub-liquid chamber 128 are disposed under the elastic body 116 with a partition wall member 124 interposed between the main liquid chamber 126 and the first sub-liquid chamber 128. The main liquid chamber 126 and the first sub-liquid chamber 128 are connected with each other by a first orifice 130 formed in the partition wall member 124. Further, a second sub-liquid chamber 142 connected with the main liquid chamber 126 by a second orifice 144 is formed within the partition wall member 124.
In other words, the vibration isolating apparatus 200 is constructed in that a plurality of liquid chambers 126, 128 and 130 are provided in addition to the elastic body 116 and the liquid chambers 126, 128 and 130 are connected by a plurality of orifices 130, 144 each serving as a restricting passage. When the engine mounted on the vibration isolating apparatus 200 operates and vibration is generated, vibration is absorbed by a damping function of the elastic body 116, viscous resistance of a liquid within the orifices 130, 144 provided for communication between the liquid chambers 126, 128 and 130, or the like. As a result, transmission of vibration is prevented.
However, in the vibration isolating apparatus 200, a diaphragm 132 allowing the capacity of the first sub-liquid chamber 128 to vary is disposed at a lower side of the first sub-liquid chamber 128 and a diaphragm 146 is needed at a lower side of the second sub-liquid chamber 142 as well. For this reason, in the vibration isolating apparatus 200, it becomes necessary to provide the second sub-liquid chamber 142 and the diaphragm 146 within the partition wall member 124, and the structure of the partition wall member 24 becomes complicated. As a result, there exists a drawback in that the cost of manufacturing the vibration isolating apparatus 200 will increase.
On the other hand, in the vibration isolating apparatus 200 as described above, the length of the second orifice 144 in the longitudinal direction thereof which is used to reduce vibration in an idle vibration range cannot be sufficiently obtained and vibration in an idle vibration range cannot be sufficiently reduced. As a result, directly-opposed characteristics of a shake vibration range and an idle vibration range cannot be sufficiently satisfied. Further, when vibration of a higher frequency range is generated by the engine, the vibration cannot be reduced sufficiently.
In the conventional vibration isolating apparatus described hereinbefore, there exist drawbacks in that vibrations of a wide range of frequencies cannot be reduced sufficiently and a large vibration is transmitted to the vibration-receiving portion depending on the frequency of vibrations.